Chakraborty Saheli, Halat David M, Im Julia, Hickson Darby T, Reimer Jeffrey A, Balsara Nitash P
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
Department of Chemical and Biomolecular Engineering, University of California, Berkeley, Berkeley, California 94720, USA.
Phys Chem Chem Phys. 2023 Aug 9;25(31):21065-21073. doi: 10.1039/d3cp00923h.
One approach for improving lithium transference in electrolytes is through the use of bulky multivalent anions. We have studied a multivalent salt containing a bulky star-shaped anion with a polyhedral oligomeric silsesquioxane (POSS) center and lithium counterions dissolved in a solvent. The charge on each anion, , is equal to -20. The self-diffusion coefficients of all species were measured by pulsed field gradient NMR (PFG-NMR). As expected, anion diffusion was significantly slower than cation diffusion. An approximate transference number, also referred to as the current fraction (measured by Bruce, Vincent and Watanabe method), was higher than those expected from PFG-NMR. However, the rigorously defined cation transference number with respect to the solvent velocity measured by electrophoretic NMR was negative at all salt concentrations. In contrast, the approximate transference numbers based on PFG-NMR and current fractions are always positive, as expected. The discrepancy between these three independent approaches for characterizing lithium transference suggests the presence of complex cation-anion interactions in solution. It is evident that the slow self-diffusion of bulky multivalent anions does not necessarily lead to an improvement of lithium transference.
一种改善电解质中锂迁移的方法是使用体积较大的多价阴离子。我们研究了一种多价盐，其包含一个带有多面体低聚倍半硅氧烷（POSS）中心的体积较大的星形阴离子以及溶解在溶剂中的锂抗衡离子。每个阴离子上的电荷，(z)，等于 -20。所有物种的自扩散系数通过脉冲场梯度核磁共振（PFG-NMR）进行测量。正如预期的那样，阴离子扩散明显慢于阳离子扩散。一个近似迁移数，也称为电流分数（通过布鲁斯、文森特和渡边方法测量），高于PFG-NMR预期的值。然而，通过电泳核磁共振测量的相对于溶剂速度的严格定义的阳离子迁移数在所有盐浓度下均为负。相比之下，基于PFG-NMR和电流分数的近似迁移数如预期的那样始终为正。这三种用于表征锂迁移的独立方法之间的差异表明溶液中存在复杂的阳离子 - 阴离子相互作用。显然，体积较大的多价阴离子的缓慢自扩散不一定会导致锂迁移的改善。
